Rock-top defence
Ideally starting with a far area horde set-up (but by all means try it with any set-up), drive or run the short distance to the large rock just around the corner to the right, then clamber up on top and attack the enemy from there. You should have a fine view of the covies across the plain, and can have fun whittling them down bit by bit, hopefully making good use of your grenades along the way.
A pistol and sniper rifle would make the ideal combination for this, but you can also have fun with a needler. Plasma weapons can do a good job also, though a plasma rifle won't last terribly long. It's not an intense battle for the most part, but it's got some novelty value.
See if you can do some sneaky tagging. It can actually be quite easy because of how the covies casually stand around, especially if they've temporarily lost sight of you, at which point they may be looking away.
Be on the lookout for good enemy concentrations to frag too, such as you often get in or near the small semicircular area formed by low rocks beneath the huge rock just across the way (see second pic).
Need a suitable rock
This plan depends on the rock geometry of course. You'll need a set-up in which this particular rock is climbable. In the set-up I was using for the pictures here, the rock had a nice sloping face on my side of it, which not only let me climb up but also gave good cover, It was easy to back off or duck down for a while, to recharge my shield when needed. And it certainly was needed sometimes. A lot of enemy fire can come your way, sending loads of rubble flying off your rock!